1. Field of the Invention
This disclosure relates to a direct current circuit breaker, and particularly, to a current detecting mechanism capable of detecting a ground current.
2. Background of the Invention
A high voltage direct current (abbreviated as HVDC) transmission is an electric power transmission method of converting high voltage alternating current (abbreviated as AC hereinafter) electric power (voltage) generated in a generating station into high voltage direct current (abbreviated as DC hereinafter) electric power (voltage) using an electric power converter for transmission and thereafter reconverting the DC voltage into the AC voltage in an electric power-receiving area for supply.
The DC transmission method has an advantage of facilitating device insulation because a maximum value of a DC voltage corresponds to 70% of an AC voltage, and also reducing the quantity of insulators installed in each device and a height of a steel tower for transmission because of a low voltage. Also, when transmitting the same electric power, a power transmission loss in the DC transmission is less than that in the AC transmission. Accordingly, the DC transmission may improve transmission efficiency and reduce the amount of lines used and an area of transmission line. Owing to those advantages, the DC transmission is expected to be increasingly applied later in all parts of the world.
The present disclosure relates to a DC circuit breaker as electrical equipment for DC transmission.
A detection of an amount of electric current (hereinafter, referred to as a detection of electric current) flowing through the DC circuit breaker is the basis for executing a function of the DC circuit breaker of breaking an electrical circuit when a fault current such as a short circuit current, or overcurrent, or ground fault current is generated on the electric circuit. Therefore, it is a very important function in the DC circuit breaker.
In general, an AC circuit breaker facilitates for measuring an AC current using a current transformer or a Rogowski coil sensor. However, the DC circuit breaker is not easy to measure a DC current due to impossibility of measuring a DC current using an induction by an alternating magnetic flux because the DC current is not alternating.
Hereinafter, disclosure will be given of an example of measuring a DC current in a DC circuit breaker according to the related art with reference to FIGS. 1 and 2.
As shown in FIGS. 1 and 2, a DC circuit breaker according to one example of the related art includes a cradle 12 providing an enclosure, and a breaker main body 10 having a pair of wheels 1 at both lower sides, respectively, and movable to an drawn-in position or a drawn-out position with respect to the cradle 12.
The cradle 12 is a member which provides the enclosure of the DC circuit breaker. The cradle 12 may be provided with a door 11 on its front surface to be opened and closed, and a terminal part 14 configured, for example, as a conductive bus bar to which electric power lines of a electric power source side and an electric load side are connectable.
The DC circuit breaker according to the one example of the related art includes a DC shunt 13 located at the rear of the cradle 12 and electrically connected to the terminal part 14 for detecting a DC current on a DC electric circuit.
When a DC current flows on the terminal part 14 via the electric line connected to the terminal part 14 of the rear of the cradle 12, the corresponding DC current also flows on the DC shunt 13 connected to the terminal part 14 and a voltage in proportion to the flowing DC current is generated across the DC shunt 13. As the corresponding voltage is measured, the DC current may be measured.
Although not shown, a controller (a measuring and control unit), such as an overcurrent relay, is installed at the door 11. The controller, such as the overcurrent relay, determines an occurrence of a fault current such as overcurrent or shortcircuit current based on the DC current on the electric circuit detected by the DC shunt 13, and control the breaker main body 10 to break the circuit.
However, the DC current detecting mechanism for the DC circuit breaker according to the one example of the related art is configured such that the DC shunt 13 is separately fabricated and connected to the terminal part 14 of the rear of the cradle 12. This increases the size of the DC circuit breaker, which makes it difficult to realize size reduction of the DC circuit breaker. Also, as the DC shunt 13 is additionally installed, costs and assembling time are required and an increase in fabricating costs and lowered productivity of the DC circuit breaker are caused.
Hereinafter, disclosure will be given of another example of detecting a current flowing through a DC circuit breaker with reference to FIG. 3.
In FIG. 3, a reference numeral 100 designates a breaker main body as a main component of the DC circuit breaker.
The DC circuit breaker may include a plurality of main switch units depending on a magnitude of a rated voltage. FIG. 3 exemplarily shows that four main switch units 110, 120, 130, 140 are connected in series to one another. For example, in a DC circuit breaker that a rated voltage is 1000 volts (V), the main switch units 110, 120, 130, 140 are assigned with 250V, respectively.
A DC circuit breaker may also be configured according to another example in which only two of four main switch units 110, 120, 130, 140 are connected to be assigned with 250 V of a rated voltage of 500 V, respectively.
Each of the main switch units 110, 120, 130, 140 may include a movable contact arm 103, a fixed contact arm (not shown), and an arc extinguishing mechanism (not shown) for arc extinguishing.
Referring to FIG. 3, the main switch units 110, 120, 130, 140 may be connected in series by a pair for each of anode and cathode, namely, a pair of main switch units 110, 120 and another pair of main switch units 130, 140 are connected to each other in series.
The breaker main body 100 may further include a switching mechanism (not shown) for simultaneously switching on or off the main switch units 110, 120, 130, 140.
The main switch units 110, 120, 130, 140 may include electric power source side terminals 100a1, 100a3, electric power source side common terminals 100a2, 100a4, electric load side terminals 100b2, 100b4, and electric load side common terminals 100b1, 100b3. Here, the electric power source side common terminals 100a2, 100a4 and the electric load side common terminals 100b1, 100b3 may be used as terminals for electrically connecting the electric power source side terminals to the electric load side terminals without an electrical connection to an external line of an electric power source side or an electric load side.
The pair of main switch units 110, 120 and the pair of main switch units 130, 140, each of which is in the connected state in series, may be electrically connected to each other by a connection conductor 100c, respectively.
Anode and cathode of a DC electric power source side may be connected to the electric power source side terminal 100a1 and the electric power source side terminal 100a3, and anode and cathode of a DC load side may be connected to the load side terminal 100b2 and the load side terminal 100b4.
In the meantime, as a member for detecting a current flowing through the DC circuit breaker according to the related art, a first DC shunt 150a and a second DC shunt 150b may be installed on conducting paths via the load side terminal 100b2 and the load side terminal 100b4, respectively. The first and second DC shunts 150a and 150b may output voltage signals, each of which is in proportion to an amount of current flowing along the conducting paths via the load side terminal 100b2 and the load side terminal 100b4, as output signals.
Here, one of the first and second DC shunts 150a and 150b may merely be installed to detect the amount of electric current flowing through the DC circuit breaker. However, in order to detect a ground fault and/or a ground fault current, two DC shunts such as the first and second DC shunts 150a and 150b have to be installed on the conducting paths of the anode and cathode, respectively.
Although not shown, one end of a signal line may be connected to the first and second DC shunts 150a and 150b and the other end of the signal line may be connected to the measuring and control unit such as the overcurrent relay (not shown).
Hereinafter, disclosure will be given of an operation of detecting a current, an operation of determining whether or not a ground fault has occurred and/or detecting a ground current with reference to FIG. 3.
As shown in FIG. 3, anode and cathode of a DC electric power source may be connected to the electric power source side terminal 100a1 and the electric power source side terminal 100a3, respectively.
A DC current may flow from the anode electric power source side 100a1 toward an electric load through the main switch unit 120 and the anode load side terminal 100b2 via the main switch unit 110 in the closed state and the connection conductor 100c. The DC current may then flow from the electric load side into the cathode load side terminal 100b4, and then flow into the main switch unit 130 and the cathode electric power source side terminal 100a1 via the main switch unit 140 in the closed state and the connection conductor 100c. 
Here, each of the first and second DC shunts 150a and 150b may output a voltage signal which is proportional to the DC current flowing toward the load side or the DC current flowing from the load side.
The output voltage signal may then be transmitted to a microprocessor of the measuring and control unit such as the overcurrent relay which is connected via the signal line. The corresponding microprocessor may convert the received voltage signal into a current according to a predetermined ratio of current to voltage and a conversion program, and measure an amount of current flowing through the DC circuit breaker.
An operation of detecting whether or not a ground fault has occurred will be described as follows.
When a ground fault has not occurred on a circuit connected to the DC circuit breaker, the first DC shunt 150a installed on the conducting path connected to the anode load side terminal 100b2 in series and the second DC shunt 150b installed on the conducting path connected to the cathode load side terminal 100b4 in series may output voltage signals which have the same value with different signs to transmit to the microprocessor of the measuring and control unit. The microprocessor may then add the output voltages (i.e., obtain a sum of vectors) to obtain a result of 0 (zero). Here, the microprocessor may decide non occurrence of the ground fault.
When the ground fault has occurred on the circuit connected to the DC circuit breaker, the first DC shunt 15 installed on the conducting path connected to the anode load side terminal 100b2 in series may output an output voltage corresponding to the current. However, since the fault current (ground current) is introduced into the cathode DC electric power source side via the ground of a frame of the DC circuit breaker, an output voltage of the second DC shunt 150b may be a value whose absolute value is different from the output voltage of the first DC shunt 150a. 
Hence, the microprocessor of the measuring and control unit such as the overcurrent relay connected to the first and second DC shunts 150a and 150b may add the output voltages output from the first and second DC shunts 150a and 150b to obtain a value which is proportional to the ground current, other than 0 (zero). Accordingly, the microprocessor may measure the amount of ground current and decide the occurrence of the ground fault.
However, in the related art, the output voltage signals of the first DC shunt 120a and the second DC shunt 120b are very high voltages of an electric power system. Accordingly, those output voltage signals may not be applied to the measuring and control unit as they are, but should be applied via an insulation and step-down transformer. Therefore, the insulation and step-down transformer has to be equipped.
Especially, to deal with a voltage of a DC electric system reaching up to 1000 Volt, the insulation and step-down transformer has to be designed to tolerate a voltage more than 1000 Volt. This may result in an increase in size of the DC circuit breaker and fabricating costs thereof.